You can crossfire any similar card (eg. 4800 series with another 4800).
The cards will use as much RAM as the lowest of the pair.
However, the cards will not down clock to the slowest of the pair.
The cards will run asynchronously, though this mode isnt very efficient. Based on numbers I have seen around the performance of mixed crossfire is only slightly better than the performance of twin crossfire with two of the slowest card.
However, in teh 3800 series things were a bit different. a 3870 + 3850 crossfire woudl fit nicely in performacne between a 3870x2 and 3850x2. This is not the case, for whatever reason, in any data I have seen with the 4800 series. Far as cost effectiveness goes it is not really worth corssfiring anything but identical cards and the improvements of the faster card are minimal.